The electrodeposition of zinc-cobalt alloys on metallic substrates such as iron, steel, and like metals to provide increased corrosion resistance is finding increasing acceptance in the marketplace. Such alloys not only provide increased corrosion resistance compared to traditional zinc deposits, but have the additional advantage of exhibiting bright, aesthetically pleasing surfaces.
Illustrative of electrolytes for electroplating of zinc-cobalt alloys from acid solution are those described in U.S. Pat. No. 4,325,790 and British Patents 2,116,588A and 2,160,223A. However, the metal concentration in such electrolytes is relatively high, which makes waste water treatment expensive and time-consuming. Further, the content of cobalt in the alloys deposited from these electrolytes is a function of the cathode current density. Shaped parts are, therefore, difficult to coat uniformly using this type of electrolyte.
Electrolytes for plating zinc-cobalt deposits from alkaline media (i.e., pH of 8-9) are also known. See, for example, U.S. Pat. No. 4,717,458, which employs a chelating agent such as sodium glucoheptonate in combination with salts of zinc and cobalt. The high content of chelate and of cobalt salt in the elecrolyte makes expensive and time-consuming the treatment of waste water in an environmentally acceptable manner.
Other electrolytes containing complexing agents are described, for instance, in U.S. Pat. No. 4,299,671 in which the pH of the electrolyte is in the range of 6-9 and complexing agents such as citric, gluconic, glucoheptonic, and tartaric acids are employed. Ligands such as ethylenediamine, diethanolamine, and triethanolamine can also be used in the alkaline electrolyte baths.
The properties of these zinc-cobalt coatings (alloy composition, corrosion resistance) are not as good as those of the coatings deposited from the electrolytes proposed herein. The complexes of the noted ligands with cobalt salts are unstable and they precipitate in the course of electrolysis upon being added into an alkaline electrolyte and after the lapse of time. In addition, treatment of waste liquids from such baths is similarly expensive and time-consuming.
It has now been found that the use of novel cobalt salt complexes in an electrolyte for electrodeposition of zinc-cobalt not only serves to obviate the above problems, but also gives rise to improved efficiency of the electroplating process and improved properties of the cobalt-zinc alloy which is deposited.